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Revision as of 04:37, 5 May 2003
Paper is a thin, flat material produced by the compression of fibres invented by Ts'ai Lun in AD 105. The fibres used are usually natural and based upon cellulose. The most common material is wood pulp from softwood trees such as pines, but other materials including cotton (flax) and hemp may be used.
Manufacture
The paper making process has four simple steps:
Preparation of the fibres:
The material to be used for making the paper needs to be made into a concentrated solution containing the fibres, called pulp. As many of these fibres are derived from natural sources, this requires many stages of separation and washing. Once the fibres have been extracted, they may also be bleached or dyed to alter the appearance of the final product.
Forming into sheets:
The pulp now needs to be formed into the desired shape. This can be achieved by a mould or by a continuous rolling process. A watermark may be impressed into the paper at this stage of the process.
In the case of the mould process, a quantity of the pulp is placed into a form, with a wire-mesh base (or other draining device), such that the fibres are left coated on the mesh, and excess water can drain away. At this time pressure may be applied to removed more water through a squeezing action. The paper may then be removed from the mould, wet or dry, and go on to further processing.
Modern, mass produced paper will be made using a continuous rolling process. The mould gauze is replaced with a gauze conveyor belt, and the paper is passed through successive rollers that apply more and more pressure to remove the water.
Further additives:
Raw paper, that contains only pressed and dried pulp, is very absorbent (for example, blotting paper), and does not provide a good surface upon which to write or print. Thus, a huge variety of additives are employed to add desired properties to the paper. These are applied in a coating called the size.
Sizing agents are often polymers designed to provide a better printing surface. Starches are very commonly used, as is PVA, but there as many types of polymer employed as there are types of paper.
Sizing agents can also seek to improve the printing surface by smoothing it. The texture of raw paper is rough, and so to achieve greater smoothness, a sizing agent such as clay is used. Smooth, matt finish papers, such as magazine paper (for the inside pages) is made in this way. The glossy effect (for example on the covers of fashion magazines) is achieved at the end of the printing process, by adding a clear layer (like varnish) over the printing, and so is not a property of the paper.
Other additives are employed to enhance various properties of the paper, the most common of which are optical brighteners.
Drying
The paper may actually be dried several times during its manufacture (dry paper is much stronger than wet, so it is best to keep the paper dry to prevent it breaking and stopping the production line).
Applications
- to write or print on
- for packaging
- for cleaning:
- toilet paper
- handkerchiefs
- miscellanious cleaning in the kitchen, etc.
See also: cardboard, ISO 216, papyrus, paper sizes